The oil and gas industry uses various tools to probe the formation penetrated by a borehole in order to locate hydrocarbon reservoirs and to determine the types and quantities of hydrocarbons. Among these tools, sonic tools have been found to provide valuable information regarding formation properties. In sonic logging, a tool is typically lowered into a borehole, either after the well has been drilled or while the well is being drilled, and sonic energy is transmitted from a source into the borehole and surrounding formation. The sonic waves that travel in the formation are then detected with one or more receivers.
A typical sonic log can be recorded on a linear scale of slowness versus depth in the borehole, and is typically accompanied by an integrated-travel-time log in which each division indicates an increment of one microsecond per foot of the total travel time for the sonic wave to travel one foot. Sonic logs are typically used as direct indications of subsurface properties or—in combination with other logs or other data of the subsurface properties—to determine the formation porosity and other parameters which cannot be measured directly.
Various analysis methods are available for deriving formation properties from the sonic log data. Among these, the slowness-time-coherence (STC) method is commonly used to process the monopole sonic signals for coherent arrivals, including the formation compressional, shear, and borehole Stoneley waves. See U.S. Pat. No. 4,594,691 issued to Kimball et al. and Kimball et al., Geophysics, Vol. 49 (1984), pp. 274-281.
For logging-while-drilling (LWD) sonic logging, it is desirable to send selected data uphole or wherever desired in real-time via mud pulse telemetry. Mud telemetry is a common method used in LWD operations to transmit log data to the surface. Mud telemetry makes use of the modulations of the pressure of a drilling fluid pumped through the drilling assembly to drill the wellbore. The fluid pressure modulation, however, can only transmit data at a rate of a few bits per second. A typical LWD sonic job requires too much bandwidth to transmit all the desired measured sonic data in real-time.
The limitations imposed on data transmission by a lack of adequate bandwidth are commonly encountered in various logging operations, not just sonic logging. Therefore, various methods for data compression have been developed to reduce the bandwidth requirement of conventional telemetry schemes. For example, U.S. Pat. No. 5,381,092 issued to Freedman describes methods for compressing data produced from NMR well tools. The methods first subdivide a plurality of input signals into multiple groups, where the number of groups is much less than the number of input signals. The method then generates one value for each group. Thus a plurality of values corresponding to the plurality of groups represent the compressed input signals transmitted uphole.
U.S. Pat. No. 5,031,155 issued to Hsu describes methods for compressing sonic data acquired in well logging. Samples of each digitized formation wave component are characterized as a vector. Eigenvectors based on the formation wave component vectors are obtained, and selected wave components are correlated to the eigenvectors to obtain scalar correlation factors. The eigenvectors and correlation factors together provide a compressed representation of the selected formation wave component.
U.S. Pat. No. 6,691,036 issued to Blanch et al. describes methods for processing sonic waveforms. A method proposed in this application transforms an acoustic signal into the acoustic frequency domain to produce an acoustic frequency domain semblance and display the result in a graph with slowness and acoustic frequency axes. Published U.S. Patent Application No. 2004/0145503 by Blanch et al. describes additional methods for processing sonic waveforms.
U.S. Pat. No. 6,405,136 B1 issued to Li et al. describes compression methods for use in wellbore and formation characterization. The method includes performing a 2D transform on the data in the orientation domain and in a domain related to the recording time.
While these methods are useful in compressing log data and in reducing the bandwidth requirements of mud telemetry, a need remains for efficient techniques for downhole data compression.